Datasheet

OPA843
12
SBOS268C
www.ti.com
WIDEBAND INVERTING SUMMING AMPLIFIER
One common application for a wideband op amp like the
OPA843 is to sum a number of signal sources together.
Figure 5 shows the inverting summing configuration that is
most often used. This circuit offers the benefit that each input
sees an input impedance set only by its individual input
resistor, since the summing junction (inverting op amp node)
is a virtual ground. Each input is non-interactive with every
other. However, the bandwidth from any input to the summed
output is set by the op amp noise gain (NG), which is equal
to the noninverting voltage gain. Therefore, each inverting
channel may have a low gain to the output (like the –1 shown
in Figure 5); this noise gain will set the frequency response
and the loop stability. The noninverting gain for Figure 5 is
equal to +5, which will give a 260MHz bandwidth at a gain of
–1 for each of the input signals.
transition from a unity gain receiver at lower frequencies
(through the R
5
path) to a gain of 20dB (10V/V) through the
R
1
path at higher frequencies. The component values have
been selected to set the peak gain at approximately 30MHz.
A unique feature for this circuit is an independent tune on the
width of the peaking (Q of the response) by adjusting R
G
.
See Figure 9 for the effect of adjusting R
G
over the range of
20Ω to 100Ω.
DESIGN-IN TOOLS
DEMONSTRATION FIXTURES
Two printed circuit boards (PCBs) are available to assist in
the initial evaluation of circuit performance using the OPA843
in its two package options. Both of these are offered free of
charge as unpopulated PCBs, delivered with a user's guide.
The summary information for these fixtures is shown in the
table below.
2nd-Order Filter Topology
High-speed amplifiers like the OPA843 are good choices for
2nd-order filter building blocks as part of ADC driver chan-
nels. These can provide noise bandlimiting to improve the
SNR for the amplifier/converter combination. The circuit of
Figure 6 shows an example of a 10MHz Butterworth low-
pass filter where the amplifier provides a low frequency gain
of 5 and a 2nd-order cutoff at 10MHz. The resistor values
have been adjusted slightly to account for the amplifier
bandwidth. Figure 7 shows the small-signal frequency re-
sponse for this filter.
EQUALIZING FILTER APPLICATION
In sensor receiver applications, where the pickup is a sensor
or cable giving a bandlimited frequency response, an equal-
izing filter can sometimes be used to extend the useable
frequency range for the sensor. This is done mathematically
by taking the inverse of the rolloff transfer function and
implementing that as the amplifier frequency response. See
Figure 8 for one example of a wideband equalizer where two
stages of the OPA843 are used. This example is set to
The demonstration fixtures can be requested at the Texas
Instruments web site (www.ti.com) through the OPA843
product folder.
R
F
402Ω
OPA843
+5V
–5V
0.1µF
402Ω
81.8Ω
V
O
= –(V
1
+ V
2
+ V
3
+ V
4
)
V
1
402Ω
V
2
402Ω
V
3
402Ω
V
4
Power-supply decoupling not shown.
FIGURE 5. Wideband Inverting Summing Amplifier.
OPA843
150Ω
100pF
61Ω
0Ω
Source
100Ω
402Ω
220pF
V
I
V
O
Frequency (MHz)
10MHz Low-Pass Filter
100k 1M 10M 100M
Gain (dB)
15
12
9
6
3
0
–3
–6
–9
–12
–15
FIGURE 6. 10MHz Butterworth Low-Pass Filter.
FIGURE 7. Frequency Response for Figure 6.
ORDERING LITERATURE
PRODUCT PACKAGE NUMBER NUMBER
OPA843U SO-8 DEM-OPA-SO-1A SBOU009
OPA843N SOT23-5 DEM-OPA-SOT-1A SBOU010